1. Field of the Invention
The invention relates to a process for the treatment in a reactor of spent pot linings derived, in particular, from the dismantling of cells for the production of aluminum by electrolysis according to the Hall-Heroult technique.
2. Description of Related Art
An aluminum production plant with a capacity of 240,000 metric tons per year produces approximately 4,000 metric tons per year of spent pot linings which are composed of carbon-containing cathodic blocks, seals, and lateral linings fabricated of carbon-containing paste, and also of all the refractories and insulators disposed on the lateral walls and the bottom of the metallic tank forming the electrolytic cell. After use, these products of the lining are strongly impregnated with noxious substances, such as soluble sodium fluorides, sodium aluminates, and cyanides, that must be insolubilized or destroyed before discharge or re-use.
Processes for treatment of old pot linings by a wet process, consisting of grinding followed by leaching in an alkaline solution, have already been described, for example in U.S. Pat. No. 4,052,288 (Aluminium Pechiney) or U.S. Pat. No. 4,113,631 (Kaiser).
These processes, which necessitate making use of expensive means, are limited solely to the treatment of carbon-containing parts of pot linings, which should be separated beforehand from the non-carbon-containing parts formed by the refractories and insulators.
Other known processes include treatment by a thermal process operating generally in a fluidized bed, and based either on pyrolysis of the spent pot linings at more than 1000.degree. C., in accordance with U.S. Pat. No. 4,065,552 (Elkem) or U.S. Pat. No. 4,113,832 and U.S. Pat. No. 4,115,908 (Kaiser), or on simple combustion in the air or in an oxidizing atmosphere of carbon-containing elements at a temperature of approximately 800.degree. C., which is sufficient to decompose the cyanides without provoking a major discharge of volatile fluorine-containing compounds, in accordance with U.S. Pat. No. 4,053,375 (Reynolds) or according to the article by L. C. Blayden and S. G. Epstein, Journal of Metals, Jul. 1984, page 24.
In fact, all the processes and devices using thermal process are limited in their possibilities by the nature and the composition of the linings to be treated. Indeed, because of the melting of certain eutectic compounds formed in the course of combustion, the pot lining particles have a strong tendency to agglomerate. It quickly becomes impossible to prevent the particle from agglomerating, and subsequently, to maintain a fluidized and dense bed if the combustion is achieved, for example, in a rotary kiln with a relatively long holding time. This phenomenon of agglomeration, already perceptible in the batches of pot linings constituted solely by carbon-containing products, is strongly accentuated in the batches containing refractory oxides, especially silica, of which the gravimetric content must not exceed 3 or 4%, as can be learned from the article by E. R. Cutshall and L. O. Daley, Journal of Metals, Nov. 1986, page 37, Table II.
The variant, consisting of achieving combustion in a reactor in a circulating fluidized bed of carbon-containing pot linings in a mixture with an additive composed of finely ground refractory oxides, in accordance with U.S. Pat. No. 4,763,585 (Ogden), limits the risks of superficial bonding followed by the agglomeration of the particles. In order to do this, the additive must remain inert during the combustion, that is to say neither reacting nor melting in contact with the carbon-containing wastes in a temperature range generally set on the order of 1000.degree. C. However, here again, the problem of agglomeration is not resolved, except insofar as one treats the carbon-containing pot linings, or at least the pot linings derived from the linings with very low silica content, therefore essentially aluminum-based, whose life span is clearly inferior to that of modern linings in silico-aluminous preforms.
A process perfected by the present applicant and described in an earlier patent application (EP-A 0 485 388, corresponding to U.S. Ser. No. 07/716,104) permits treating all types of pot linings, especially those in which the gravimetric content of silica can reach 50%, without prior sorting and under conditions offering complete environmental safety, with total decomposition of cyanides, whose content in spent pot linings can reach 1%, and with substantially complete insolubilization of alkaline fluorides, which can represent up to 20% fluorine and 20% sodium by weight when the carbon content is generally between 20% and 40%.
This process comprises heating the spent pot linings abruptly, and for a very short time, at a temperature sufficient to permit the decomposition of cyanides, and preferably in the presence of an additive that is pulverulent and reactive, that is to say able to combine chemically with the fluorides impregnating the pot linings, in order to form stable, insoluble compounds such as CaF.sub.2 ; the binary, ternary or quaternary compounds of NaF, CaF.sub.2, CaO, SiO.sub.2, Al.sub.2 O.sub.3, CaSO.sub.4, Na.sub.2 SO.sub.4 ; and nepheline, hauynite, or other types of compounds. This abrupt heating for a very short time enables the avoidance of all combustion of the carbon-containing products, which also remain in the form of stable, insoluble compounds.
More precisely, the mixture of ground pot linings with the pulverulent additive, preferably kaolin, anhydrous or hydrous CaSO.sub.4, CaO or a mixture thereof, is injected into the upper part of a reactor in a circulating gaseous flow at a temperature of between 1100.degree. C. and 1800.degree. C., and the time of contact between solid particles in the mixture and the circulating gaseous flow is regulated so that the particles reach a temperature of at least 750.degree. C. before their extraction at the base of the reactor with the gaseous flow. In steady operation, the temperature of the gaseous flow, measured at a point in the reactor at a set value, is regulated by adjusting the gravimetric output of the pulverulent mixture injected at the top of the reactor. Exiting at the base of the reactor, the gaseous flow, after cooling, is separated from the solid particles constituting the stabilized residue, that is to say the residue in which the noxious compounds have been insolubilized or destroyed.